1. Geology 212 Petrology Prof. Stephen A. Nelson
Metamorphic Rock Textures
This document last updated on 20-Feb-2003
Metamorphic rocks exhibit a variety of textures. These can range from textures similar to the
original protolith at low grades of metamorphism, to textures that are purely produced during
metamorphism and leave the rock with little resemblance to the original protolith. Textural
features of metamorphic rocks have been discussed in the previous lecture. Here, we
concentrate on the development of foliation, one of the most common purely metamorphic
textures, and on the processes involved in forming compositional layering commonly observed
in metamorphic rocks.
Foliation
Foliation is defined as a pervasive planar structure that results from the nearly parallel
alignment of sheet silicate minerals and/or compositional and mineralogical layering in the
rock. Most foliation is caused by the preferred orientation of phylosilicates, like clay minerals,
micas, and chlorite. Preferred orientation develops as a result of non-hydrostatic or differential
stress acting on the rock (also called deviatoric stress). We here review the differences
between hydrostatic and differential stress.
Stress and Preferred Orientation
Pressure increases with depth of burial, thus, both pressure and temperature will vary with
depth in the Earth. Pressure is defined as a force acting equally from all directions. It is a type
of stress, called hydrostatic stress or uniform stress. If the stress is not equal from all
directions, then the stress is called a differential stress. Normally geologists talk about stress as
compressional stress. Thus, if a differential stress is acting on the rock, the direction along
which the maximum principal stress acts is called σ1, the minimum principal stress is called σ3,
and the intermediate principal stress direction is called σ2. Note that extensional stress would
act along the direction of minimum principal stress.
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2. If differential stress is present during metamorphism, it can have
a profound effect on the texture of the rock.
Rounded grains can become flattened in the direction of
maximum compressional stress.
Minerals that crystallize or grow in the
differential stress field may develop a
preferred orientation. Sheet silicates and
minerals that have an elongated habit will
grow with their sheets or direction of
elongation orientated perpendicular to the
direction of maximum stress.
This is because growth of such minerals is easier along directions parallel to sheets, or
along the direction of elongation and thus will grow along σ3 or σ2, perpendicular to σ1.
Since most phyllosilicates are aluminous minerals, aluminous (pelitic) rocks like shales,
generally develop a foliation as the result of metamorphism in a differential stress field.
Example - metamorphism of a shale (made up initially of clay minerals and quartz)
Shales have fissility that is caused
by the preferred orientation of clay
minerals with their {001} planes
orientated parallel to bedding.
Metamorphic petrologists and
structural geologists refer to the
original bedding surface as S0.
Slate Slates form at low metamorphic grade by the growth of fine grained chlorite and
clay minerals. The preferred orientation of these sheet silicates causes the rock to easily
break planes parallel to the sheet silicates, causing a slatey cleavage.
Note that in the case shown
here, the maximum
principle stress is oriented at
an angle to the original
bedding planes so that the
slatey cleavage develops at
an angle to the original
bedding. The foliation or
surface produced by this
deformation is referred to
S1.
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3. Schist - The size of the mineral grains
tends to enlarge with increasing grade
of metamorphism. Eventually the rock
develops a near planar foliation caused
by the preferred orientation of sheet
silicates (mainly biotite and
muscovite). Quartz and feldspar
grains, however show no preferred
orientation. The irregular planar
foliation at this stage is called
schistosity
Gneiss As metamorphic grade increases, the sheet silicates become unstable and dark
colored minerals like hornblende and pyroxene start to grow.
These dark colored minerals tend to
become segregated into distinct
bands through the rock (this process
is called metamorphic
differentiation), giving the rock a
gneissic banding. Because the dark
colored minerals tend to form
elongated crystals, rather than sheet-
like crystals, they still have a
preferred orientation with their long
directions perpendicular to the
maximum differential stress.
Granulite - At the highest grades of metamorphism most of the
hydrous minerals and sheet silicates become unstable and thus there
are few minerals present that would show a preferred orientation.
The resulting rock will have a granulitic texture that is similar to a
phaneritic texture in igneous rocks.
In general, the grain size of metamorphic rocks tends to increase with increasing grade of
metamorphism, as seen in the progression form fine grained shales to coarser (but still fine)
grained slates, to coarser grained schists and gneisses.
Metamorphism and Deformation
Most regionally metamorphosed rocks (at least those that eventually get exposed at the Earth's
surface) are metamorphosed during deformational events. Since deformation involves the
application of differential stress, the textures that develop in metamorphic rocks reflect the
mode of deformation, and foliations or slatey cleavage that develop during metamorphism
reflect the deformational mode and are part of the deformational structures.
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4. The deformation involved in the formation of
fold-thrust mountain belts generally involves
compressional stresses. The result of
compressional stress acting on rocks that
behave in a ductile manner (ductile behavior is
favored by higher temperature, higher confining
stress [pressure] and low strain rates) is the
folding of rocks. Original bedding is folded
into a series of anticlines and synclines with
fold axes perpendicular to the direction of
maximum compressional stress. These folds
can vary in their scale from centimeters to
several kilometers between hinges. Note that
since the axial planes are oriented perpendicular
to the maximum compressional stress direction,
slatey cleavage or foliation should also develop
along these directions. Thus, slatey cleavage or
foliation is often seen to be parallel to the axial
planes of folds, and is sometimes referred to
axial plane cleavage or foliation.
Metamorphic Differentiation
As discussed above, gneisses, and to some extent schists, show compositional banding or
layering, usually evident as alternating somewhat discontinuous bands or layers of dark colored
ferromagnesian minerals and lighter colored quartzo-feldspathic layers. The development of
such compositional layering or banding is referred to as metamorphic differentiation.
Throughout the history of metamorphic petrology, several mechanisms have been proposed to
explain metamorphic differentiation.
1. Preservation of Original Compositional Layering. In some rocks the compositional
layering may not represent metamorphic differentiation at all, but instead could simply be
the result of original bedding. For example, during the early stages of metamorphism and
deformation of interbedded sandstones and shales the compositional layering could be
preserved even if the maximum compressional stress direction were at an angle to the
original bedding.
In such a case, a foliation might
develop in the shale layers due to
the recrystallization of clay
minerals or the crystallization of
other sheet silicates with a
preferred orientation controlled by
the maximum stress direction.
Here, it would be easy to determine that the compositional layers represented original
bedding because the foliation would cut across the compositional layering.
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5. In highly deformed rocks that have
undergone both folding and shearing,
it may be more difficult to determine
that the compositional layering
represents original bedding. As
shearing stretches the bedding,
individual folded beds may be
stretched out and broken to that the
original folds are not easily seen.
Similarly, if the rock had been injected by dikes or sills prior to metamorphism, these
contrasting compositional bands, not necessarily parallel to the original bedding, could be
preserved in the metamorphic rock.
2. Transposition of Original Bedding. Original compositional layering a rock could also
become transposed to a new orientation during metamorphism. The diagram below
shows how this could occur. In the initial stages a new foliation begins to develop in the
rock as a result of compressional stress at some angle to the original bedding. As the
minerals that form this foliation grow, they begin to break up the original beds into small
pods. As the pods are compressed and extended, partly by recrystallization, they could
eventually intersect again to form new compositional bands parallel to the new foliation.
3. Solution and Re-precipitation. In fine grained metamorphic rocks small scale folds,
called kink bands, often develop in the rock as the result of application of compressional
stress. A new foliation begins to develop along the axial planes of the folds. Quartz and
feldspar may dissolve as a result of pressure solution and be reprecipitated at the hinges
of the folds where the pressure is lower. As the new foliation begins to align itself
perpendicular to σ1, the end result would be alternating bands of micas or sheet silicates
and quartz or feldspar, with layering parallel to the new foliation.
4. Preferential Nucleation. Fluids present during metamorphism have the ability to
dissolve minerals and transport ions from one place in the rock to another.
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6. Thus felsic minerals could be
dissolved from one part of the
rock and preferentially nucleate
and grow in another part of the
rock to produce discontinuous
layers of alternating mafic and
felsic compositions.
5. Migmatization. As discussed previously, migmatites are small pods and lenses that
occur in high grade metamorphic terranes that may represent melts of the surrounding
metamorphic rocks. Injection of the these melts into pods and layers in the rock could
also produce the discontinuous banding often seen in high grade metamorphic rocks. The
process would be similar to that described in 4, above, except that it would involve
partially melting the original rock to produce a felsic melt, which would then migrate and
crystallize in pods and layers in the metamorphic rock. Further deformation of the rock
could then stretch and fold such layers so that they may no longer by recognizable as
migmatites.
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